1. Field of the Invention
The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing a conformal layer of material on a substrate in an ionized metal plasma process.
2. Background of the Related Art
Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductor and other electronic devices, directionality of particles being deposited on a substrate is important to improve adequate filling of electric features. As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to 0.25 .mu.m or less, whereas the thickness of the dielectric layer remains substantially constant. Thus, the ratio of the depth to the minimum lateral dimension (aspect ratio) of the features increases, thereby pushing the aspect ratios of the vias and contacts to 5:1 and above. As the dimensions of the features decrease, it becomes even more important to get directionality in order to achieve conformal coverage of the feature sidewalls and bottoms.
One process capable of providing greater directionality to particles is ionized metal plasma-physical vapor deposition (IMP-PVD), also known as high density physical vapor deposition (HDP-PVD). Initially, a plasma is generated by introducing a gas, such as helium or argon, into the chamber and then applying a negative voltage to a metallic target. The voltage will cause the gas to form a plasma. A coil positioned proximate the processing region of the chamber, and powered with RF energy, produces an electromagnetic field which induces currents in the plasma resulting in an inductively-coupled medium/high density plasma between the target and a susceptor on which a substrate is placed for processing. The positive ions in the plasma are accelerated toward the target by the applied bias causing the sputtering of material from the target by momentum transfer. The sputtered target material is then ionized by interaction with the plasma, thereby producing metal ions in the case of a target comprising a metal. An electric field due to an applied or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate that accelerates the resultant metal ions towards the substrate in a direction generally parallel to the electric field and perpendicular to the substrate surface exposed to the processing region. The bias energy is preferably controlled by the application of power, such as RF or DC power, to the susceptor to attract the sputtered target ions in a highly directionalized manner to the surface of the substrate to fill the features formed on the substrate. HDP-PVD is particularly beneficial in depositing material on the bottom of the features due to the influence of the substrate bias energy on the target ions. In order to maximize the deposition rate, it is preferred to maximize the substrate bias energy.
One of the problems with HDP-PVD processes is that the applied substrate bias energy also attracts the inert plasma gas ions toward the substrate to cause re-sputtering of the deposited material from the substrate. Re-sputtering of deposited material can result in a phenomenon known as "faceting" whereby the deposited material at the upper end of a feature formed in the substrate is partially etched by the ions from the plasma gas. Under the influence of the substrate bias, the charged particles can gain enough kinetic energy to re-sputter a portion of the deposited material. In particular, the material is re-sputtered from the top comers of the device features, such as vias, and from the fields of the substrate, which are the upper planar areas of the substrate formed between the features. The re-sputtered material then re-deposits on other areas of the substrate, particularly on the upper portion of the features formed on the substrate.
Faceting is undesirable because of the potential to form overhangs as illustrated in FIGS. 1-2. FIG. 1 is a cross section of a via 30 formed on a substrate 31. The via 30 is formed in a dielectric layer 32 having a layer 34 disposed thereon, which may be a barrier layer or a seed layer for example. During a conventional HDP-PVD process, a bias on the substrate 31 results in an electric field E oriented substantially perpendicularly to the substrate 31. The electric field E accelerates the nearby positively charged particles toward the substrate 31. While the ions generated from the sputtered material (shown here as metal ions) deposit on the dielectric layer 32 to form the layer 34, the plasma gas ions (shown here as Ar ions) re-sputter the deposited material and produce overhangs 36 which restrict the opening of the via 30. The overhangs 36 are shown as portions of the deposited layer 34 at the upper end of the via 30 which are relatively thicker compared to the other portions of the layer 34 due to greater deposition at those locations on the via 30. As the overhangs 36 continue to accumulate material, entry to the via 30 is restricted and a void 38 is formed, as shown in FIG. 2.
Re-sputtering occurs above a threshold ion energy value below which no re-sputtering occurs. The ion energy is dependent on the voltage drop in the plasma sheath. Metals typically re-sputter at ion energy thresholds that occur between about 20V and 40V. Above the threshold, the re-sputter yield, i.e., the number of metal atoms ejected from the substrate per incident Ar ion, increases with increasing ion energy. Because the ion energy is dependent on the voltage drop in the plasma sheath, re-sputtering is increased as the substrate bias is increased. In most cases, some degree of re-sputtering is acceptable until the ratio of re-sputtering to deposition reaches a critical limit. Accordingly, the maximum value of the bias applied to a substrate is limited before producing detrimental effects such as faceting. As a result, the ability of HDP-PVD processes to control deposition with the bias energy is compromised.
Further, the occurrence of re-sputtering above a threshold ion energy value is related to the process pressure maintained in the chamber. In general, higher pressures result in more ionization of the target material and hence, more particles that can be influenced by the substrate bias. HDP-PVD processes typically require a process pressure between about 15 mTorr and 100 mTorr in order to ensure sufficient ionization of the sputtered target material. However, higher process pressures also result in a higher ratio of the plasma gas ion flux to the metal ion flux due to the greater proportion of plasma gas ionization as compared to metal ionization. As described above, the plasma gas ions are the agents responsible for re-sputtering. Therefore, as the ratio of process gas ions to metal ions increases, the maximum value on the substrate bias energy decreases before re-sputtering occurs.
Therefore, there is a need for a method of depositing a layer conformally over the surface of features wherein overhangs can be controlled and good bottom coverage can be achieved.